1. Field of the Invention
This invention generally relates to magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena. More particularly, this invention is directed to novel MRI magnet structures for use in an MRI system.
2. Description of the Prior Art
A great variety of MRI systems are now commercially available. They vary greatly in design criteria as well as in purchase, installation and operating costs. They also have considerably different technological strengths and weaknesses with respect to patient comfort, image quality, patient accessibility by attending physicians--and many other differences that will be readily understood by those skilled in the art of designing MRI systems.
As many in the art have already recognized, if MRI is to be increasingly utilized, then further improvements in the economics of MRI are highly desirable. At the same time, technological features related to image quality cannot be unduly sacrificed to economic goals. Since many MRI system design features simultaneously relate to many different desirable and undesirable MRI system features, it is quite difficult to find a truly optimum MRI system design that simultaneously tends toward all the desired goals (including lower economic costs).
At the present time, commercial MRI systems typically can be classified into two groups: (a) those using a large electromagnet to create the static background field B.sub.o and (b) those using a large permanent magnet structure to create the necessary static background field B.sub.o. Although there are some resistive electromagnets with air and iron cores in use, most MRI electromagnets use low T.sub.c superconductive electromagnet solenoidal windings with an air core. On the other hand, the permanent magnet MRI systems typically utilize a large number of high strength permanent bar magnets arrayed symmetrically about upper and lower pole pieces located above and below an air gap in which the usual patient imaging volume is symmetrically located. In the latter permanent magnet MRI systems, the return magnetic circuit is typically completed with a magnetically permeable yoke. Other designs of rings of permanent magnet materials have been put forward but without commercial success; these yokeless permanent magnet systems will not concern us here.
The air core superconductive MRI electromagnet structures taken as one class are compared to a typical permanent MRI magnet system in the table below:
TABLE I ______________________________________ Permanent Magnet Air Core (e.g., see Superconductive U.S. Pat. No. 4,829,252 - Electromagnet Kaufman) ______________________________________ (a) Advantages 1. Moderate to high field 1. low cost at low fields strength at reasonable costs 2. high stability 2. Open accessible in magnetic field patient volumes strength B.sub.o 3. no Hysteresis in 3. Eddy currents can be magnetic circuit controlled (See USSN (although this is 07/546,112 - Kaufman no longer true for et al above) newer self-shielded magnets 4. No cryogen services required (b) Disadvantages 1. Tunnel-like patient 1. Hysteresis in magnetic volumes circuit 2. Helium Consumption 2. Poor stability (cost/convenience) of magnetic field strength B.sub.o 3. Eddy currents in 3. Costs increase conductive metal rapidly with cryostat increased field strength 4. Costs do not decrease at low fields. ______________________________________
Of course one would like to find a way to simultaneously achieve all of the advantages of both MRI system types described above in Table I while eliminating all of their combined disadvantages Unfortunately, it is not easy to approach such an ideal design.
For example, one of the reasons that costs increase rapidly with increased field strength in permanent magnet systems is the cost, weight and volume of required permanent magnet material used to achieve such high field strength. If one considers using electromagnets instead to generate the driving magnetic field, then it is perhaps reasonable to expect that increased field strength might be achieved without such rapidly increasing economic costs. However, if the electromagnet driver is to use a resistive winding, there would be necessary added capital and operating costs involved in the need for continuously supplied electrical power (both for the magnet winding itself and for a suitable cooling system) as well as the need for cooling water and the like. There may also be additional problems encountered with stability.
On the other hand, if a conventional low T.sub.c superconducting electromagnet is to be used as a substitute for the permanent magnets, then it would inherit many of the disadvantages now associated with air core superconductive electromagnet MRI designs. For example, one would then encounter the cost and inconvenience of continuous helium cryogen consumption and the like. Furthermore, costs would not substantially decrease for low field designs using such conventional low T.sub.c superconducting drivers.
While others appear to have recognized the possibility that high T.sub.c superconducting materials may have applications in the MRI context as a means of replacing the low T.sub.c materials in currently available magnets, none are believed to have yet recognized that high T.sub.c materials may be utilized as the superconducting electromagnet windings for an MRI magnet design of open architecture resembling, in many advantageous respects, presently available permanent magnet MRI system designs. In short, it is not believed that others have yet recognized that high T.sub.c superconducting materials may be advantageously employed to effectively combine many of the advantageous features of presently existing disparate MRI magnet designs.
Some examples of prior suggestions for use of high T.sub.c superconducting materials in the MRI context are set forth below:
In general, when high T.sub.c superconductors were discovered, there was considerable initial excitement in the technological community--including the MRI design community. Although initial commentaries typically included MRI as one of the fields that might be revolutionized by high T.sub.c superconducting materials (presumably because MRI was known to be a user of low T.sub.c superconducting materials), the design community tended to treat the topic somewhat conservatively (e.g., see the above noted references 2-4).